1. Technical Field of the Invention
The present invention relates to a mobile radio system and, in particular, to the duplication and communication of subscriber data among home location registers within a cellular telecommunications system.
2. Description of Related Art
A home location register (HLR) is a data base which serves mobile switching centers in a mobile telephone system. A mobile telephone system may comprise one or more such HLR units. Each HLR unit contains information regarding all the mobile telephone subscribers that belong to the geographic area that is covered by the mobile switching centers (MSC) connected to the HLR unit. For instance, this data base contains a record for each connected subscriber in the mentioned geographic area. Each such record further contains different types of subscriber information, for instance billing information, available subscriber feature services, etc. Such information is called the subscriber categories. Moreover, each record contains information on which MSC a particular subscriber is currently associated. This location information is required to enable redirection of incoming calls intended for that subscriber to the MSC in the coverage area of which the subscriber is currently located.
Whenever an incoming call is received for a particular subscriber or the subscriber moves into a different MSC coverage area, the corresponding HLR needs to be updated and queried to properly process the call. Consequently, if a particular HLR should fail or restart, all associated subscribers for that HLR would be without telecommunications service. Accordingly, maintaining the integrity and reliability of the HLRs and the data stored therein is essential for proper operation of a cellular telecommunications system.
Typically, when a particular HLR fails or restarts, back-up information is retrieved from an auxiliary storage medium and loaded onto the failed HLR to replace and correct the corrupted data. However, there exists data which might not be replaceable by the auxiliary storage medium. For example, that information which changes constantly cannot be stored permanently in a backup database. Such information includes the roaming locations of some of the subscribers. When that kind of data is lost due to an HLR failure, it needs to be retrieved or reproduced within the telecommunications network.
One method of replacing such information is to inform all visitor location registers (VLR) that this particular HLR has failed and that the failed HLR needs to be updated with new location data for subscribers that are roaming in each VLR coverage area. An example of such an operation is the "Reset" operation which is signaled over the Signaling System No. 7 (SS7) network (Unreliable Roamer Data Directive in IS-41). Such procedures, however, take time and further strain the telecommunications network as the failed HLR and the system are trying to recover from the fault. In the meantime, mobile subscribers remain without telephone service and some of the important subscriber data are also permanently lost.
A number of systems have been developed to alleviate the above problems. One such system updates the backup database much more frequently to reduce the amount of data that are lost when an HLR fails. Even though such a system might reduce the data loss, the failed HLR still has to be restarted and mobile subscribers are still without telecommunications service until the HLR recovers. Another system discloses dual HLR databases to replace and support each other when one of the two fails. The data stored in one HLR is mirrored by the other. Whenever one of the two HLRs fails, the other surviving HLR automatically takes over the functionality of the failed HLR and provides service in real time to the subscribers associated with the failed HLR. Accordingly, the failure of one of the HLRs is unnoticed by the associated subscribers and no telecommunications service is disrupted. However, such a system often requires a direct communication link between the two HLRs to communicate data and to control the operation of each other. Because of such physical connection requirements, the two HLRs have to be located in the proximity of each other. Additionally, a separate communication link and its interface module need to be established within each HLR.
An application for patent related to this application and filed Jan. 12, 1995, Ser. No. 08/372,074 (hereinafter referred to as the '074 application) discloses a system where two HLRs support each other via a SS7 telecommunications network without the need for additional links or interface modules between the two mated HLRs. However, the system disclosed in the '074 application has certain system limitations. The two HLRs, as disclosed in the '074 application, need to be connected through the same two Signal Transfer Points (STPs). Each node in a SS7 telecommunications network is supported by dual STPs. In case the first STP or links between the first STP and the destination node fails, the second STP is utilized to provide reliable network operation. The system as disclosed in the '074 application requires that the two HLRs providing support for each other be connected to the same two STPs. This implies that the two HLRs have to be physically limited within the same geographic region. Instead of associating with a most suitable HLR anywhere in the SS7 network, two HLRs within the same region are used to support each other. If one of the HLRs has no database space or processor time, because of such restrictions, the data support is not possible.
Also, the '074 application does not specify an addressing or signaling scheme for instructing a slave HLR to recognize rerouted signals intended for a failed master HLR. Without such addressing or signaling scheme to allow the slave HLR to recognize and accept the rerouted signals, the slave HLR will fail to recognize signals rerouted to the slave HLR as correctly delivered signals. This is because the Signal Point Codes (SPC) within the rerouted signals do not correspond with the SPC for the slave HLR.
Another concern with the '074 application relates to addressing when a master HLR communicates data to its slave HLR. If the master HLR transmits a signal toward the slave HLR, and if the slave HLR has failed or is unreachable, the signals are automatically rerouted back to the master HLR by the system STPs. In other words, the network cannot, without a specific addressing scheme, currently distinguish signals communicated between the two HLRs and signals received from other nodes over the network. As a result, if a HLR fails, the '074 system always reroutes the signals to the slave HLR of the failed HLR. In this particular case, that slave HLR is the original HLR who has initially generated the rerouted signal. By returning the same signal to the original HLR, if the data contents of the original HLR have changed in interim, the data integrity of the original HLR will be inadvertently destroyed.
Accordingly, it would be advantageous to enable two HLRs located anywhere within the Signaling System No. 7 (SS7) telecommunications network to support each other in real time without requiring additional communications links between the two and without destroying the integrity of the data base.